1. Field of the Invention
This invention relates to four-wheel drive, hydraulically driven vehicles and a control system to equalize the torque between front and rear drive wheels.
2. Description of the Prior Art
Heavy vehicular equipment designed primarily for off-road duty is commonly hydraulically driven. One such vehicle, used in seismic exploration for oil, is disclosed in U.S. Pat. No. 3,905,466, assigned to the assignee of this invention. Such vehicles have two or more axles, each of which supports a pair of single or dual traction wheels, generally rubber-tired. Each pair of wheels is driven by a hydraulic motor thorugh a differential reduction gear.
The hydraulic motors are supplied with hydraulic drive fluid from a hydraulic pump. The pump, in turn, is usually powered by a prime mover such as a gasoline or diesel-fueled engine.
In the case where a single pump supplies two or more hydraulic motors, if one traction wheel spins out, all of the hydraulic fluid is diverted to the motor that drives the spinning wheel. No power reaches the other wheels and consequently the vehicle is stalled. Accordingly, each motor is driven by a separate pump. The two pumps of course, may be driven from a common drive shaft of the prime mover.
When two independent pumps deliver hydraulic power to the motors that drive the traction wheels, the hydrostatic drive pressures in the two systems must be the same. In the absence of pressure equalization between the two systems, the drive system having the greater hydrostatic pressure will develop more torque; the associated traction wheels will turn faster than the wheels associated with the system having the lesser torque. The slower pair of traction wheels will apply a counter torque to the corresponding drive motor which then tends to act as a dynamic brake. The slower wheels will drag to cause excessive tire wear, destruction of the roadway, and overall loss of power.
Many factors contribute to an unbalance in the hydrostatic drive pressure, for example, uneven tire sizes, differential wheel rotation during turns, mechanical inaccuracies in matching two different pumps or motors, and unequal-length plumbing runs to the two systems wherein fluid friction impedes the flow of hydraulic fluid in the longer hydraulic line.
As an example of a mechanical problem, in one vehicle, it was found that the same pilot pressure applied to the swash plate servo positioners of two different pumps, imparted a one-half degree greater tilt to the front pump swash plate than to that of the rear pump. Parenthetically, it may be noted that a "swash plate" is employed in a hydraulic pump to control, the pump displacement and hence the volume (and pressure) of hydraulic fluid applied to the motors. A change in the angle of the swash plate controls the pump displacement by changing the stroke of pump pistons mounted in a rotating cylinder block. In the example at operating pressure, the one-half degree difference in the swash plate tilt changed the pump displacement so that the front pump discharged 1.5 gallons per minute more fluid than the rear pump. The front drive motor accordingly rotated 63 revolutions per minute (rpm) faster than the rear motor. Reduced by the differential gear ratio of 19:1 the front wheels rotated 3.32 rpm faster than the rear wheels. The above situation resulted in a severe power loss and roadway damage. To overcome the problem the hydrostatic drive pressures in the two systems had to be equalized.
One method for pressure equalization is disclosed in U.S. Pat. No. 3,724,583, with particular reference to FIG. 9 and Col. 9, lines 1-27 of the patent. In this patent, a pressure equalizing spool valve is connected between the two hydraulic drive systems. An excess of pressure in one system causes the spool to move in the direction of lower pressure, which opens a port to allow the fluid having the higher pressure to bleed into the system having the lower pressure.
There are disadvantages to the above-cited pressure equalization system. The drive pressure lines are directly interconnected with each other through the valve. If one set of wheels slips, the other set will lose power as described earlier for the case of two motors and one pump.
For an equalizer valve of manageable size, the various passageways in the valve and valve spool must be relatively restricted. At a normal operating pressure of several thousand pounds per square inch, and with the volume of fluid that must flow back and forth between the two drive systems of the reference, a very considerable amount of undesirable heat will be developed. Furthermore, no means is disclosed for damping movement of the spool to prevent hunting.
Finally, the system disclosed in the patent is a "brute force" device. Fine control of the pressure balance between systems is not possible.